Modern aircraft presently use elastomeric and other polymeric matrix materials in a wide variety of applications including seals, hoses, tires, O-rings, etc., and many of these parts must operate over a very wide temperature range and at high pressures, some parts encountering pressures up to 5000 psi and temperatures in a range from -65.degree. to 800.degree. F. These elastomeric parts are reinforced by inorganic fibers which must be compatible with the matrix material, and must have maximum strength to extend the life of the parts in the pressure and temperature ranges in which they operate. Inadequate strength at elevated temperatures has been a limiting physical characteristic for elastomeric materials. These materials for many aircraft applications must not only be stable in a variety of environments at elevated temperatures but also must maintain adequate strength.
For example, hydrofluorocarbon elastomers have exhibited superior aging qualities in the temperature range of 400.degree. to 600.degree. F, but the use of these elastomers has been severely hampered because of their low strength at the elevated temperatures. It has been demonstrated that the addition of compatible inorganic fibrous reinforcing materials to fluoroelastomers can markedly increase tensile strength at 400.degree. F and improve their resistance to strength degradation after prolonged periods at 600.degree. to 800.degree. F. It has also been shown that purity of the reinforcing media, which is required to preserve compatibility, is necessary to achieve maximum resistance to high temperature degradation. Fluoride fibers have been used with hydrofluorocarbon elastomers, and metal oxide fibers have been used to reinforce other elastomers and polymers as well as hydrofluorocarbon elastomers.
It is generally recognized that as the diameter of a fiber is reduced, assuming substantially circular cross-section, the tensile strength of the fiber is increased, the tensile strength varying as the ratio of the circumference to the mass. In order to achieve higher strength products, it is therefore desirable to be able to make smaller diameter reinforcing fibers while still preserving their purity and their microcrystalline structure.
The prior art U.S. Pat. Nos. 3,385,915 to Hamling, and 3,832,451 to Abrams and Shaver teach processes for producing various inorganic fibers by impregnating organic precursor fibers with solutions of salts and then calcining the impregnated precursors to drive off the organic matter and leave inorganic fibers of microcrystalline structure. In general, the impregnating solutions used in the prior art were rather concentrated so as to leave in the precursor fibers a dense deposit of salts, whereby the diameter of the resulting mineral fiber was closely related to the diameter of the precursor fiber, the shrinkage usually not exceeding about 40% to 60% as compared with the diameter of the original precursor fiber. The degree of loading of the salts into the precursor fiber according to the teachings of the prior art was deliberately increased by pre-swelling of the precursor by soaking it in water. U.S. Pat. No. 3,385,915 (supra) includes the statement that the degree of shrinkage is inversely proportional to the degree of loading of metal compound into the precursor, see column 7, lines 68-70; and further includes lines 44 to 59, which indicates that there is a minimum impregnation loading below which inadequate strength of the resulting mineral fiber will result.
The accepted way of controlling the diameter of the finished mineral fiber has been to select a suitable diameter for the precursor fiber, such that after calcining of the loaded precursor and sintering of the resulting mineral fiber, the latter will have the desired diameter. In practice, however, this approach fails to provide small enough mineral fibers, because very small precursor organic fibers are not available as a manufactured fiber, and natural organic fibers have larger diameters. The smallest diameter rayon precursor fiber tested was 0.75 denier per filament (d.p.f.), which was used to produce a MgO fiber averaging 8 microns in diameter, but even this 0.75 d.p.f. fiber has now become commercially unavailable. Moreover, it was not a very uniform diameter fiber as to diameter. Currently, rayon fibers of 1.1, 1.5 and 3.0 d.p.f. are the smallest available diameters. The 1.1 d.p.f. fibers average about 11 microns in diameter, although they vary between 8 and 14 microns in commercially available products. This variation is excessive.
In order to produce mineral fibers of smaller diameters at acceptable costs, one must use commercially available precursor organic fibers, and therefore work was done to provide an improved mineral-fiber-making process which could impregnate lower salt concentrations into available organic precursor fibers of the order of 1 d.p.f. without the degradation in crystal structure or in strength of the resulting mineral fiber, this effort being contrary to the degradation forecast in U.S. Pat. No. 3,385,915 as a result of lowering still further the concentration of the impregnating salt solutions.